Diffusion driven water purification apparatus and process

ABSTRACT

An apparatus for purifying water, such as for desalinization, includes a source of a heated air stream, the heated air stream having a temperature above an ambient temperature. A diffusion tower having high surface area material therein receives a water stream including at least one impurity and creates at least one region having thin films of water therein from the water stream. The heated air stream is directed over the thin films of water to create a humidified air stream that is at least substantially saturated. At least one direct contact condenser is in fluid communication with the humidified air stream for condensing the humidified air stream, thus producing purified water. A power plant can include the apparatus for purifying water, where energy to heat the air stream is provided by low pressure condensing steam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.10/322,061 entitled “DIFFUSION DRIVEN DESALINATION APPARATUS ANDPROCESS” filed on Dec. 17, 2002, now U.S. Pat. No. ______, and isincorporated by reference into the present application in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD OF THE INVENTION

The invention relates to water purifications systems and methodsincluding desalination.

BACKGROUND

The continuous rise in the world population and the expansion ofindustrial facilities around the globe has placed a growing demand onthe fresh water supply from natural resources, such as rivers, freshwater lakes, underground aquifers, and brackish wells. These resourceshave been steadily on the decline since the early 1950's. Therefore,there is clearly a need for new fresh water resources to balance thegrowing consumption rate.

Since 96% of the earth's surface is covered with saline water, there hasbeen and continues to be strong motivation for developing waterdesalination technologies. Today there are more than 7,500 desalinationplants in operation worldwide, and about two thirds of those areoperating in the Middle East. Saudi Arabia operates the largestdesalination plant, with a capacity of 128 MGD. The United Statesaccounts for about 12% of the world's desalination capacity.

Desalination involves any process in which dissolved minerals areremoved from saline or brackish water. Technologies for desalinationinclude distillation, reverse osmosis, electro-dialysis, and vacuumfreezing. Distillation and reverse osmosis are the most common.Distillation technologies include Multiple Effect Distillation (MED) andMulti-Stage Flashing (MSF), both of which operate by evaporating salinewater at atmospheric or reduced pressure and condensing the vapor toproduce fresh potable water. Reverse Osmosis (RO) operates on afiltering principle. High pressure pumps force saline water throughnanofilter membranes allowing fresh water to pass while filtering outthe dissolved minerals. Although distillation and reverse osmosistechnologies currently provide the most cost effective method fordesalination, they are both very energy intensive. Accordingly, whetheror not these desalination techniques remain cost effective stronglydepends on energy prices.

A desalination technology that has drawn interest over the past twodecades is referred to as Humidification Dehumidification (HDH).According to the HDH process, saline water is pumped through a condensercoil, where it picks up heat from condensing water vapor. The salinewater is then pumped through a solar collector where it collects moreheat. The saline water is then sprayed in a cooling tower, where aportion of it evaporates into the air. The water vapor is then condensedover the condenser coil of a conventional tube condenser. An advantageof this type of technology is that it permits low pressure, lowtemperature desalination. El-Bourouni et al. (2001), El-Hallaj et al.(1998), and Assouad et al. (1988) respectively reported the operation ofHDH units in Tunisia, Jordan, and Egypt, respectively.

Another type of desalination technology that makes use of waterevaporating into an air stream is the Carrier-Gas Process (CGP) reportedby Larson et al. (1989). A CGP system consists of two chambers separatedby a common heat transfer wall. One chamber is used for evaporation andthe other for condensation. Ambient air is directed through theevaporation chamber and mixed with high salinity feed water. The airpicks up heat from the heat transfer wall and increases in temperatureas it moves through the evaporation chamber. Some of the feed waterevaporates into the air and the rest is removed as concentrate. Thehumidified air is heated in a heater and is then sent to a condensationchamber where the water vapor is condensed out. Purified water iscollected in the condensation chamber.

Even though both HDH and CGP may provide some level of improvedefficiency compared to more conventional desalination methods, HDH andCGP are both still energy intensive and provide limited water productionefficiency (e.g. kilograms of fresh water per kilogram of feed water).Accordingly, even these improved methods still generally have limitedapplication.

SUMMARY OF THE INVENTION

A diffusion driven desalination (DDD) process and apparatus is driven bywater vapor saturating low humidity air. Liquid water is then condensedout of the air/vapor mixture in a condenser. The invention is suitablefor operation at low temperature and pressure and may be driven by wasteheat with low thermodynamic availability. The energy consumption for theDDD process is comparable to that for conventional flash evaporation andreverse osmosis processes.

An apparatus for purifying water includes a structure for receiving aheated water stream and creating at least one region having a thin filmof water from the heated water stream. A structure for forcing a lowhumidity air stream over the thin film of water is also provided whichcauses water from the thin film of water to evaporate and diffuse intothe air stream to create a humidified air stream. As used herein, thephrase “low humidity air stream” refers to an air stream which includesless humidity as compared to the “humidified air stream” formed byevaporation and diffusion of water into the low humidity air stream. Atleast one condenser is provided for condensing the humidified airstream, wherein purified water is obtained. The process described aboveperformed by the claimed apparatus is referred to herein as a DiffusionDriven Desalination (DDD) process.

The heated water stream can be heated at least in part by at least oneheat exchanger. The heated water stream can be supplied by a variety ofheat sources, including low pressure condensing steam from a powerplant, waste heat from a combustion engine, and geothermal heat. Thecondenser preferably comprises a direct contact condenser.

The structure for creating regions having thin films of water preferablycomprises a diffusion tower. The diffusion tower can include at leastone plenum for drawing the humidified air stream out from the diffusiontower so that it can be condensed. The plenum includes at least oneinlet, the inlet at least in part facing a top of the diffusion tower.This orientation helps prevent the heated water stream from entering theplenum.

The structure for forcing an air stream over the thin film of water cancomprise an air duct positioned near a bottom of the diffusion tower,the air duct including a plurality of holes, the plurality of holesfacing a side or a bottom of the diffusion tower, such as transverse toa height of the diffusion tower.

The condenser can provide at least a portion of the low humidity airstream by dehumidifying air using a cooling water stream. Providedsufficient thermal stratification exists, a single body of water canprovide both the heated water stream and the cooling water stream bydrawing the heated water stream from a surface of the body of waterwhile the cooling water stream is drawn from a depth below the surface.In one embodiment of the invention, the heated water stream can beprovided exclusively by a body of water so that no external heat sourceis required to provide the heated water stream. In another embodiment ofthe invention, heat to produce the heated water stream is supplied by asolar source.

A power plant including a desalination system includes an apparatus forconverting a source of energy into heat and at least a portion of theheat into steam, and structure for converting cool water supplied into aheated water stream using at least a portion of the steam. A structurefor creating regions having thin films of water from the heated waterstream is provided along with structure for forcing a low humidity airstream over the thin film of water, wherein water from the thin film ofwater evaporates and diffuses into the low humidity air stream to createa humidified air stream. At least one condenser is provided forcondensing the humidified air stream, wherein purified water isproduced. The condenser preferably comprises a direct contact condenser.

The source of energy comprises at least one fossil fuel or at least onenuclear fuel. The structure for creating regions having thin films ofwater preferably comprises a diffusion tower. The diffusion tower caninclude at least one plenum for drawing the humidified air stream outfrom the diffusion tower. The structure for forcing an air stream overthe thin film of water can comprise an air duct positioned near a bottomof the diffusion tower, wherein the air duct includes a plurality ofholes which preferably facing a side or a bottom of the diffusion tower.

A method for purifying water includes the steps of providing a heatedwater stream including at least one non-volatile impurity, the heatedwater stream having a temperature above an ambient temperature. A heatedwater stream is sprayed onto a high surface area material, wherein thinfilms of water form on surfaces of the high surface area material. A lowhumidity air stream is forced over the thin films of water, whereinwater from the thin films of water evaporates and diffuses into the lowhumidity air stream to create a humidified air stream. The humidifiedair stream is then condensed to produce purified water. The non-volatileimpurity can include at least one salt, wherein the method can comprisedesalination. The spraying and forcing steps preferably take place in adiffusion tower.

A single body of water can be utilized to provide both the heated waterstream and the cooling water stream. A single body of water can also bean exclusive source for the heated water stream. Waste heat from a powerplant can also be used to provide heat to generate at least a portion ofthe heated water stream.

A diffusion tower for humidifying air includes a rigid outer shell, aninside volume of the shell including a portion filled with high surfacearea material. At least one inlet is provided for receiving a heatedwater stream, wherein at least one region having a thin film of waterforms on the high surface area material. At least one inlet is providedfor receiving a low humidity air stream, wherein the low humidity airstream is forced over the thin film of water, wherein water from thethin film evaporates and diffuses into the air stream to create ahumidified air stream. At least one plenum is disposed near a top of thediffusion tower for drawing the humidified air stream out from thediffusion tower.

The plenum includes at least one inlet, the inlet at least in partfacing a top of the diffusion tower. The structure for forcing an airstream over the thin film of water can comprise an air duct positionednear a bottom of the diffusion tower, the air duct including a pluralityof holes, the plurality of holes facing a side or a bottom of thediffusion tower. The diffusion tower preferably provides a height todiameter ratio of at least 2.

In an alternate embodiment of the invention, an apparatus for purifyingwater comprises a source of a heated air stream, the heated air streamhaving a temperature above an ambient temperature, and a diffusion towerhaving high surface area material therein for receiving a water streamincluding at least one impurity and creating at least one region havingthin films of water therein from the water stream. The heated air streamis directed over the thin films of water to create a humidified airstream that is at least substantially saturated, and generally fullysaturated. At least one direct contact condenser is in fluidcommunication with the humidified air stream for condensing thehumidified air stream, thus producing purified water. Energy to heat theheated air stream can be obtained from a variety of sources includinglow pressure condensing steam from a power plant, waste heat from acombustion engine, geothermal heat, or solar energy.

The energy is preferably waste heat generally provided in the form oflow pressure waste steam, such as available from a power plant. Thewaste steam is preferably directed to an air cooled condenser, where theair cooled condenser transfers heat from the steam to heat the air (e.gambient air) to generate the heated air stream.

The apparatus can further comprising a water cooled condenser, where thewater cooled condenser receives and further cools the waste steam aftercooling at the air cooled condenser. In a preferred embodiment, thewater stream is provided by a sea/brackish water/water reservoir sourcewhich is fluidly connected and provides cooling water for the watercooled condenser, where the water stream exits the water cooledcondenser as a heated water stream having a temperature above ambienttemperature which is supplied to the diffusion tower.

The apparatus can further comprise a fresh water storage tank forstoring purified water, where a portion of the purified water issupplied to the direct contact condenser for condensing the humidifiedair stream. In one embodiment, the direct contact condenser includes awater sprayer, wherein the direct contact condenser condenses thehumidified air stream through contact with the water spray provided bythe sprayer. A power plant can include a water purification systemincluding this alternate embodiment of the invention, the power plantincluding an apparatus for converting a source of energy into electricalenergy and waste heat, such as in the form of low pressure steam, wherethe water purification system utilizes the waste heat to heat air usedto generate purified water.

BRIEF DESCRIPTION OF THE DRAWINGS

A fuller understanding of the present invention and the features andbenefits thereof will be accomplished upon review of the followingdetailed description together with the accompanying drawings, in which:

FIG. 1 is a flow diagram for a diffusion driven desalination system andprocess according to an embodiment of the invention.

FIG. 2 is a cross sectional view of an exemplary diffusion tower,according to an embodiment of the invention

FIG. 3 illustrates examples of different types of high surface areapacking materials that may be used with the invention.

FIG. 4 illustrates the rate of entropy generation in the diffusion towerfor T_(H)=27° C.

FIG. 5 illustrates the variation of exit brine temperature with exit airtemperature for T_(H)=27° C.

FIG. 6 illustrates the fresh water production efficiency for T_(H)=27°C.

FIG. 7 illustrates the rate of entropy generation in the diffusion towerfor T_(H)=60° C.

FIG. 8 illustrates the rate of entropy generation in the diffusion towerfor T_(H)=80° C.

FIG. 9 illustrates the variation of exit brine temperature with exit airtemperature for T_(H)=60° C.

FIG. 10 illustrates the variation of exit brine temperature with exitair temperature for T_(H)=80° C.

FIG. 11 illustrates the fresh water production efficiency for T_(H)=60°C.

FIG. 12 illustrates the fresh water production efficiency for T_(H)=80°C.

FIG. 13 illustrates the rate of energy consumption for T=60° C.

FIG. 14 illustrates the rate of energy consumption for T_(H)=80° C.

FIG. 15 illustrates the rate of energy consumption on magnified scalefor T_(H)=60° C.

FIG. 16 illustrates the rate of energy consumption on magnified scalefor T_(H)=80° C.

FIG. 17 is a flow diagram schematic for a diffusion driven waterpurification system and process according to alternate embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The invention includes a Diffusion Driven Desalination (DDD) apparatusand process. The invention can provide efficiency advantages overconventional desalination technologies and is capable of providing largeproduction rates of purified water. Although the preferred applicationfor the DDD system is for desalination, the process can also purifywater which includes other non-volatile contaminants, such as a varietyof minerals including phosphates and nitrates, or metals including heavymetals.

The DDD process can utilize naturally occurring thermal energy storageprovided by most large bodies of water, where desalination is mostlikely to be applicable. Because the desalination process can beaccomplished at relatively low temperatures, inexpensive materials maybe used for constructing a processing facility, and waste heat may beutilized to drive the desalination process, such as condensing steamfrom a nuclear or fossil fuel power plant which is a potential energysource which has been previously untapped. Accordingly, the inventioncan be operated using energy which can be provided at essentially nocost, thus making the inventive process significantly more economicalwhen compared to other desalination processes.

Good performance of the process can be realized with an inlet feed watertemperature as low as about 60° C. In certain applications, thedesalination process may be driven without any heating of the inlet feedwater, such as when there is a significant temperature gradient betweenthe surface water, which can be drawn in as feed water, and the coolingwater found at lower depths. When a significant thermal gradient existsin the feed water source utilized, desalination or purification canproceed without energy provided by any external heat source. Even whenan external heat source is used, this feature of the invention allowsthe production efficiency (fresh water/feed water) of the DDD process toimprove.

One important application for the invention involves placing a DDD planton site at a steam driven electric generating plant, where the wasteheat from condensing low pressure steam may be used to drive thedesalination process. The ability to utilize waste heat from condensinglow pressure steam results in significant efficiency advantages of theinvention over conventional desalination technologies.

The following nomenclature is used in this detailed description: Acontrol surface area (m²) C_(pa) specific heat of air (kJ/kg) h enthalpy(kJ/kg) h_(fg) latent heat of vaporization (kJ/kg) m_(ll) feed watermass flow rate (kg/s) m_(a) air mass flow rate (kg/s) P_(a) partialpressure of air (kPa) R_(a) engineering gas constant for air (kJ/kg-K){dot over (s)} entropy generation rate in the diffusion tower (kW/K) Ttemperature (C. or K) V control volume (m³) ω humidity ratio ρ density

Subscripts a air fw fresh water l water in liquid phase v water in vaporphase

A simplified schematic diagram of the DDD process and system 100 isshown in FIG. 1. A main feed pump 105 draws water from a large body ofwater 110, such as seawater. The suction for pump 105 preferably drawswater near the surface of the body of water 110 in order to draw thewarmer water that normally resides in the vicinity of the surface due tothermal stratification, as compared to cooler water which resides atdeeper depths beneath the surface. The surface water is optionallypumped through regenerative heater 115 as explained below and then fedto the main feed water heater 120. The amount of heat required to heatthe feed water depends on the main feed water mass flow rate and desiredproduction rate. The output temperature from the heater is generallyrelatively low, such as from 60 to 80° C. which can provide goodperformance. Therefore, the required heat input for the heater can beprovided by a variety of sources.

The only requirement for the heat source(s) which heat the supply wateris the ability to raise the temperature of the water stream to atemperature above the ambient temperature. Sources for heating caninclude heat from low pressure condensing steam in the main condenser ofa steam driven power plant, waste heat derived from exhaust gases from acombustion engine such as a gas turbine or diesel engine, solar heating,or heat from a direct fossil fuel furnace or nuclear power plant. Inaddition, waste heat from another processing plant, such as a refineryor chemical production facility, would also generally be suitable foruse with the invention.

Solar heating is particularly suitable for desert regions (e.g. MiddleEast) since solar heating generally requires a very large area to heatsignificant quantities of water. This might be economical in the MiddleEast or other locations where there is plenty of sun and inexpensiveland.

After the feed water is heated in the main heater, it is sprayed ontothe top of the diffusion tower 125. The diffusion tower 125 is animportant piece of equipment in the process, and the degree to which anoperational DDD process follows theoretically predicted models dependson an appropriately designed diffusion tower. On the bottom of thediffusion tower 125, low humidity air is pumped in, such as by a forceddraft blower 130. The water sprayed into the top of the diffusion tower125 falls countercurrently to the airflow through the diffusion tower bythe action of gravity. The diffusion tower 125 is preferably packed withvery high surface area packing material, as would be found in aconventional air-stripping tower.

As water flows through the diffusion tower 125, a thin film of waterforms on the surfaces of the packing material. The thin film of water iscontacted by the air flowing upward through the tower 125 which ispropelled by blower 130. As dictated by Fick's law of diffusion and thelaws of conservation of mass, momentum, and energy, liquid water willevaporate and diffuse into the air, while air will diffuse into thewater, both due to concentration gradients.

The diffusion tower 125 is preferably designed such that the air/vapormixture leaving the tower should be fully saturated. The purpose ofheating the water prior to entering the diffusion tower is that the rateof diffusion and the exit humidity ratio increases with increasingtemperature, thus yielding greater water production. The water which isnot evaporated in the diffusion tower 125 is preferably collected at thebottom of the tower and can be removed with a brine pump 135. The brinecan be discharged or sent through a regenerative heater 115 for recoveryof the heat above ambient possessed by the brine. Generally, when thebrine temperature exceeds about 30° C. and/or when the brine temperatureexceeds the incoming water temperature, the brine is preferably sent toa regenerative heater, otherwise it is preferably simply discharged. Ifit is expected the feed water temperature will not exceed about 30° C.or the brine temperature will not exceed the incoming water temperature,the regenerative heater 115 is generally not included with system 100.With appropriate maintenance it is not expected that scaling of thediffusion tower 125 will pose any significant problem since the brinewill wash away residual minerals left behind by the evaporated water.

If the mineral concentration in the brine is high, there is thepotential for scaling to occur on the packing material. However, usingoperating conditions described herein, a low percentage (e.g. less thanor equal to 11%) of the feed water is evaporated which will result in alow mineral concentration in the brine making scaling of the packingmaterial only generally a minimal concern.

The air entering the diffusion tower 125 is preferably dry air. Air canbe dried in a direct contact condenser 140 for this purpose. Thesaturated air/vapor mixture leaving the diffusion tower 145 is drawninto a direct contact condenser 140 with a forced draft blower 150,where the water vapor is condensed into fresh water that is collected inthe sump 155 of condenser 140.

It may be possible to use types of condensers other than the directcontact type. However, due to the presence of non-condensable gas (air)mixed with the water vapor, a traditional shell and tube condenser wouldgenerally require excessive heat transfer area. Thus, it is anticipatedthat a direct contact condenser will yield the highest heat transferefficiency of the available condenser types.

The air stream exiting the condenser is preferably dehumidified by thecondenser cooling water (not shown) and is indicated as dry air 155. Thecondenser 140 can run in either a recirculating or non-recirculating(open) mode. In the recirculating mode, prior to being recirculated backthrough the condenser the dry air stream 155 is preferably heated backto or near ambient temperature. This can be accomplished by 1) notinsulating the return line back to the diffusion tower, 2) notinsulating the return line back to the diffusion tower 125 and putting(heating) fins on the non-insulated line to enhance heat transfer withthe assumed warmer ambient, or 3) running the dry air 155 through a heatexchanger (not shown) to pick up heat from sources including theambient.

If system 100 operates in a region where there is an abundance of dryair (low humidity), such as a desert, the non-recirculating (open) modemay be a preferred operating mode. Although not shown in FIG. 1, in theopen mode dry air is drawn into the diffusion tower 125 from the ambientand the dry air 155 exiting condenser 140 is discharged to theenvironment.

A difficulty which can arise is that film condensation heat transfer canbe substantially degraded in the presence of non-condensable gas. Thisdifficulty was faced in the design and development of condensers forOTEC (Ocean Thermal Energy Conversion) applications and the solutions tothe same may be used with the invention. For example, in order toovercome this problem Bharathan et al. (1988) discloses use ofdirect-contact heat exchangers. In their report, development of modelsis disclosed for simulating the heat transfer. Bharathan et al. havealso been awarded U.S. Pat. No. 5,925,291 for an invention entitled“Method and apparatus for high-efficiency direct contact condensation.”

For the present invention which primarily involves desalination, thewarm fresh water discharging the direct contact condenser 140 ispreferably chilled using a conventional shell-and-tube heat exchanger160 using saline cooling water. The cooling water can be drawn from alarge depth from the body of water 110 to take advantage of the thermalstratification generally present in large bodies of water. A portion ofthe chilled fresh water can be directed back to the direct contactcondenser 140 to condense the water vapor from the saturated air/vapormixture 145 which discharges from the diffusion tower 125. The rest ofthe fresh water supplied from the condenser 140 is fresh make-up water,make-up water being fresh potable water ready to be consumed or stored.Preferably, a supply of fresh potable water tanks will be located onsite for storage of fresh water produced. It is anticipated that thefresh water produced will generally be transferred via pipeline tomunicipalities or industries for consumption.

A cross-sectional view of a simplified exemplary diffusion tower 200 isshown in FIG. 2. The outer shell of the diffusion tower 205 ispreferably constructed using a mechanically strong material, such assteel-reinforced concrete. Heated feed water is pumped to a distributionheader 210 having a plurality of spray heads 211-215 located near thetop of the diffusion tower. The water is sprayed uniformly over highsurface area packing material 220. It is desirable to have a uniformwater spray because the system efficiency is enhanced by a uniformspray. In addition, the heat and mass transfer analysis used to size thediffusion tower assumes a uniform water flow over the packing material220.

Diffusion tower 200 preferably provides a minimum height to diameterratio to provide high efficiency. For example, it is recommended thatthe height to diameter ratio of diffusion tower 200 be at least 2, andmore preferably at least 10.

Low humidity air is blown into the diffusion tower through an air duct225 positioned near the bottom of the tower. The low humidity air isblown out of a duct having openings 226-229 oriented transverse to theheight of the tower. This arrangement is preferably provided so that thefalling water does not enter the air duct discharge which supplies thelow humidity air to diffusion tower 200. The air travels up through thepacking material to the top of the tower. As the air rises through tower200 it is both heated and humidified.

After the feed water is heated in the main heater, it is sprayed ontothe top of the diffusion tower 125. The diffusion tower 125 is animportant piece of equipment in the process, and the degree to which anoperational DDD process follows theoretically predicted models dependson an appropriately designed diffusion tower. On the bottom of thediffusion tower 125, low humidity air is pumped in, such as by a forceddraft blower 130. The water sprayed into the top of the diffusion tower125 falls countercurrently to the airflow through the diffusion tower bythe action of gravity. The diffusion tower 125 is preferably packed withvery high surface area packing material, as would be found in aconventional air-stripping tower.

Different types of commercially available high surface area packingmaterial 220 may be used with the diffusion tower. FIG. 3 shows examplesof some materials that may be used for this purpose. These packingmaterials are usually formed from polymers such as polyethylene, whichis suitable for the low temperature operation of the diffusion tower.Typically, a manufacturer of packing material will providespecifications regarding the surface area per unit volume. Preferredmaterials provide high surface area per unit volume ratios, such as atleast 100 m²/m³.

The height for the diffusion tower can be calculated by first choosingthe tower diameter. The diameter can be chosen such that the air flowover the packing material will be in the turbulent flow regime. Morethan one diffusion tower might be required to achieve the desired designflow conditions.

A packing material with a desired surface area per unit volume is thenspecified. A heat and mass transfer analysis is done concurrently todetermine the required tower height to achieve the design exittemperature and exit humidity ratio. Once the required height of thediffusion tower is known, a hydrodynamic analysis is done to determinethe pressure drop through the tower, which will allow the size of theforced draft blowers to be specified.

The DDD system and method is distinct as compared other desalinationprocesses, such as the carrier gas process (CGP) system and method. Oneof the major differences is that in CGP heat is added to the systemafter water diffuses into the air, and a common heat transfer wall isused for both heating and condensation. The use of the common heattransfer wall significantly reduces the area available for heat transfercompared with the packed diffusion tower and direct contact condenser.Therefore it is expected that the DDD process will have a greaterproduction efficiency than the CGP process. In addition, the CGP processdoes not recirculate the air. This will pose a significant penalty whenoperating in hot and humid climates since the relative humidity of theincoming air will be very high. Significant heating would be requiredfor the air to carry more water vapor. The DDD process can also make useof the thermal stratification in sea water while the CGP process cannot.

The DDD system and method is also distinct as compared to theHumidification Dehumidification (HDH) system and method. In the DDDprocess, the evaporation occurs in a diffusion tower as opposed to thecooling tower used in HDH. The diffusion tower is packed with highsurface area packing material, and provides significantly greatersurface area than an HDH cooling tower for the same tower size. This isimportant because the rate of water evaporation is directly proportionalto the liquid/vapor surface area available. Thus, a diffusion.,. toweris capable of a much higher rate of diffusion and thus greater freshwater production efficiency as compared to an HDH cooling tower.

Based on the considerations above, under the same operating conditions,the DDD process should have significantly greater water productionefficiency (kilograms of fresh water per kilogram of feed water) ascompared to the HDH process.

The overall DDD based desalination process requires energy consumptionthat is comparable to the energy required for flash distillation andreverse osmosis. However, a major advantage of the DDD process is thatit can operate at low temperatures so that it can be driven by an energyinput which has low thermodynamic availability. This is importantbecause the process can be driven by waste heat that would otherwise notbe suitable for doing useful work or driving some other distillationprocess, such as flash distillation. Accordingly, the invention can beoperated at least in significant part using energy which is provided atessentially no cost, thus making the inventive process significantlymore economical when compared to other desalination processes.

An important application for the DDD process is to operate inconjunction with an existing process that produces large amounts ofwaste heat and is located in the vicinity of salt water, such as anocean or a sea. One such potential benefactor of the DDD process is theelectric or utility industry. Conventional steam driven power plantsdump a considerable amount of energy to the environment via coolingwater that is used to condense low pressure steam within the maincondenser. Typically this cooling water is either discharged back to itsoriginal source or it is sent to a cooling tower, where the thermalenergy is discharged to the atmosphere. Instead of dumping the thermalenergy to the environment, the DDD process can utilize the otherwisediscarded thermal energy to produce fresh water.

The invention can be efficiently applied to power producing facilitiessited along the coastline, or along a salt or brackish water containingbody of water. This permits application at many power producingfacilities because the geographical distribution of fossil fired powerplants and nuclear power plants built in the United States and in someother industrialized parts of the world are generally proximate to largebodies of water and nearby large population bases. The demographicmake-up of the United States as well as other industrialized nations issuch that major population centers generally reside along the coastline.Thus, the DDD process appears to be well suited for the power generationinfrastructure in the United States as well as certain locations abroad.

As an example, it has been estimated that a 100 MW steam driven powerplant operating with 2″ Hg vacuum in the main condenser could provideapproximately 140 MW of energy at 93° C. available from low pressurecondensing steam (El-Walkil, 1984). If such a power plant is retrofittedor otherwise designed to operate with a DDD plant, there is thepotential to produce as much as 18 million gallons of fresh water over a24 hour period, assuming the DDD process energy consumption to be 0.05kWh/kg_(fW). The low temperature operation of the DDD process iseconomically advantageous in that inexpensive materials may be used toconstruct a facility. Since the energy required to drive the DDD processwould be free to an electrical utility, it is anticipated that thecapital investment required to fabricate a DDD plant could be readilyrecovered by selling fresh water to local industry and municipalities.

Although there exists some optimum air to feed water flow ratio thatwill minimize the energy consumption, this may not be the mosteconomical operating condition when the DDD process is driven by wasteheat. The reason is that a higher air flow rate requires more pumpingpower, which must be supplied to forced draft fans via electricity.Since electricity is a valuable commodity it may be more economical tooperate with a higher exit air temperature and a lower air to feed waterflow ratio (lower electricity consumption) since the thermal energydriving the DDD process is waste heat that would otherwise be discarded.An economic analysis, which is not considered here, may be required toidentify the optimum operating conditions based on cost considerations.

In another embodiment of the invention, a water purification system isdesigned to take advantage of air cooled condensers. Air cooledcondensers have become increasingly common, such as for power plantswhere cooling water is in short supply. As an example, the New YorkPower Authority (NYPA) has announced plans to shut down its 25-year-oldCharles Poletti Power Project in Astoria, Queens as a result of aagreement among environmental groups, the Queens Borough President andthe Power Authority. The existing facility is scheduled to be closed onFeb. 1, 2008, after a clean new 500 MW power plant proposed near thesite has begun operation. The new power plant includes an air-cooledmain condenser rather than the conventional water cooled main condenser,and thus no longer require significant water draw from the East River.

In this alternate embodiment of the invention, the inventive waterpurification system heats the air preferably using energy provided bysteam at the air-cooled main condenser of the power plant before the airenters the diffusion tower. As a result, the water entering thediffusion tower need not be heated, and as a result, a highly purifiedwater output can be obtained using much lower water flows as compared tosystem 100 shown in FIG. 1.

A schematic flow diagram for a heated air water purification system 1700according to the invention is shown in FIG. 17. For system 1700, it isassumed that the majority of low pressure “waste” steam is provided bypower plant 1701 which is condensed using an air cooled condenser 1705,with the remaining fraction of the low pressure steam being cooled in awater cooled condenser 1710. However, system 1700 can receive energy forheating from other sources (not shown), including a combustion engine,geothermal heat, or solar energy. Solar energy can be utilized, forexample, using a solar chimney where a large vertical structure absorbssolar energy and transfers energy to air flowing through the structure.

System 1700 thus includes a hybrid condenser system including a primaryair cooled condenser 1705 together with a water cooled condenser 1710which only handles a small portion of the load. The use of such a hybridcondenser system helps reduce the back pressure on the low pressureturbine in the power plant (not shown), thus improving the powerproducing capacity of the power plant. However, although not shown,system 1700 can include a single air cooled condenser.

A forced draft blower or equivalent 1712 blows ambient air through theair cooled condenser 1705, thus heating the ambient air (and cooling andcondensing the waste steam). The range of heated air temperatures isgenerally from about 40° C. to 85° C., with the preferred heated airtemperature being about 80° C. However, when used with waste heat, thewaste heat may not be at a temperature approaching 80° C. For example, apower plant will generally be operated to condense steam at as low atemperature as possible in order to maximize the power produced by theturbine. Typically this is about 50° C. With a very high vacuum thepower plant can likely get it as low as 40° C., but a temperature as lowas about 40° C. is not likely with an air cooled condenser.

The heated air exiting from the air cooled condenser 1705 is directedinto a diffusion tower 1715 which includes high surface area solidmaterial therein, such as packing material. A major advantage providedby system 1700 is that water is no longer required to heat the airprovided to diffusion tower 1715. Thus, the water flow rate into thediffusion tower 1715 may be greatly reduced, and a large fraction offeedwater may be demineralized, and in one embodiment all of the waterflow into the diffusion tower 1715 may be evaporated into the heated airprovided, thus yielding a relatively large fresh water production tofeed water ratio. An advantage to this arrangement resulting from thelow feed water flow rate into diffusion tower 1715 is that the amount ofbrine discharged by system 1700 when the source water is sea or brackishwater from the bottom of diffusion tower 1715 is greatly reduced. Asnoted above, the brine can be eliminated altogether if desired byevaporating all of the water flowing into the diffusion tower 1715, suchas through the use of a sufficiently low feed water flow.

The source water to be purified 1739 (whether sea, brackish, storm,run-off, or waste water) is drawn by a feed pump 1740 and directedthrough a fresh water chiller 1741, where the source water picks up someheat. The source water is then directed through the water cooledcondenser 1710 where it picks up additional heat from the low pressurecondensing steam as described above. The heated source water to bepurified is then preferably spayed in or proximate to the top of thediffusion tower 1715, where via through the diffusion process themajority (or all) of the source water evaporates into the heated airprovided into diffusion tower 1715. The air/vapor mixture leaving thediffusion tower 1715 is generally fully saturated. The saturated air isthen directed into the direct contact condenser 1750. Fresh waterproduction occurs as the water vapor is condensed out of the air/vapormixture by fresh water pumped into condenser 1750 from fresh waterstorage tank 1770 using fresh water pump 1775. The fresh water ispreferably sprayed using a spray head 1776. Fresh water leaving thecondenser 1750 is pumped by chiller pump 1780 to a hybrid cooling systemcomprising air cooled heat exchanger. 1781 and a water cooled heatexchanger 1741. The majority of heat is removed and discharged to theenvironment through the air cooled heat exchanger 1781.

The advantages of system 1700 as described above include:

-   -   1) system 1700 is also adapted for inland power plants in fresh        water-starved locations, and allows any mineralized water source        available to be de-mineralized with a large fresh water output.    -   2) The use of air heating allows the feed water flow rate into        the diffusion tower 1715 to be greatly reduced. This        significantly reduces the pumping energy required for the entire        process, thus making it more cost effective.    -   3) In regions were zero-waste discharge is required, the water        flow rate can be sufficiently reduced such that all of the water        entering the diffusion tower will be evaporated. This embodiment        can be useful in applications where the minerals in the water        need to be collected, concentrated, and disposed of. In this        case, the minerals, such as salts, deposit on the packing        material. When the packing gets sufficiently fouled, the packing        can be pulled out and replaced. Since the packing material is        generally made from inexpensive polyethylene matrix material        (less than $10/M³), this is not a major expense. It is possible        for the packing to be recycled to save on maintenance costs. In        this configuration, the packing acts as an evaporative filter,        where it collects and concentrates the waste product. The        environmental stewardship provided by such a process may have        many other applications outside of the power industry.    -   4) The process according to this alternate embodiment of the        invention is easily scaled up or down depending on the amount of        fresh water production required. If a reduced amount of fresh        water is required, only a fraction of the heated air flow that        the system is capable of handling from the air cooled condenser        1705 can be directed into the diffusion tower 1715.

EXAMPLES

The present invention is further illustrated by the following specificexamples. The examples are provided for illustration only and are not tobe construed as limiting the scope or content of the invention in anyway.

In order to explore the performance and parametric bounds of anexemplary DDD process, a thermodynamic cycle analysis has been performedby considering a system similar to system 100 shown in FIG. 1. Inperforming the analysis the following assumptions were made:

-   -   1. The process operates at steady-state conditions.    -   2. There are no energy losses to the environment from the heat        and mass transfer equipment.    -   3. Air and water vapor may be treated as a perfect (ideal) gas.    -   4. Changes in kinetic and potential energy are relatively small.    -   5. The pumping power is neglected in the energy balance.

In the analysis performed, the temperature of the feed water drawn intothe main feed pump was fixed at 27° C. It was also assumed that a largesupply of cool water will be available at a sink temperature. T_(L), of15° C. The condensate in the direct contact condenser was chilled and aportion of it re-circulated. To avoid providing specifics on the heattransfer equipment, it is assumed that the heat transfer effectivenessin the chiller and condenser is unity, in which case T_(L)=T₅=T₇=15° C.The temperature of the feed water leaving the main heater is the hightemperature in the system, T_(H)=T₁, and is a primary controllingvariable for the process. Different performance curves will be shown fora variable T_(H)/T_(L).

The air/vapor mixture leaving the diffusion tower is assumed to be fullysaturated (relative humidity of unity), and due to heat transferlimitations, its maximum temperature will be taken to be that of thefeed water entering the diffusion tower (T₄=T₁).

The main purpose of this analysis is to explore the performance boundsof the DDD process. However, specification of the system operatingvariables is not arbitrary. Namely, there are two constraints that mustbe satisfied, the brine temperature leaving the diffusion tower must notfreeze (T₂>0° C.), and the net entropy generation in the diffusion towermust be positive.

These constraints govern the parametric bounds for the diffusion toweroperation. While the first constraint is initially obvious, the secondconstraint is simply a restatement of the second law of thermodynamicsfor the present adiabatic system (diffusion tower). The control volumeformulation of the second law of thermodynamics for an open system isexpressed as, $\begin{matrix}{{\frac{Ds}{Dt} = {{{\frac{\partial}{\partial t}{\int_{V}{\rho\quad s\quad{\mathbb{d}V}}}} + {\oint{\rho\quad s\quad{\overset{->}{v} \cdot {\mathbb{d}\overset{->}{A}}}}}} \geq {\oint{\frac{1}{T}\frac{\overset{.}{Q}}{A}{\mathbb{d}A}}}}},} & (1)\end{matrix}$where V denotes the control volume, ρ is the density, A is the controlsurface, and s is the entropy per unit mass. Assuming steady stateprocessing of fresh water, the adiabatic diffusion tower assumptionleads to, $\begin{matrix}{{\overset{.}{s} = {\frac{Ds}{Dt} = {{\oint{\rho\quad s\quad{\overset{->}{v} \cdot {\mathbb{d}\overset{->}{A}}}}} \geq 0}}},{and}} & (2) \\{\overset{.}{s} = {{m_{12}s_{12}} + {m_{a}s_{a4}} + {m_{v4}s_{v4}} - {m_{l1}s_{l1}} - {m_{a}s_{a3}} - {m_{v3}s_{v3}}}} & (3)\end{matrix}$where m denotes the mass flow rate and the subscripts l, a and vrespectively refer to the liquid, air, and vapor phases. The numericalsubscripts denote that the property is evaluated at the statecorresponding to the position in the process as shown schematically inFIG. 1. Conservation of mass dictates that, $\begin{matrix}{\frac{m_{12}}{m_{a}} = {\frac{m_{l1}}{m_{a}} - {\left( {\omega_{4} - \omega_{3}} \right).}}} & (4)\end{matrix}$The rate of entropy generated in the diffusion tower per rate of airflow, which must be positive, is obtained from rearranging Eq. (3) andcombining with Eq. (4) as, $\begin{matrix}{{\frac{\overset{.}{s}}{m_{a}} = {{\left\lbrack {\frac{m_{l1}}{m_{a}} - \left( {\omega_{4} - \omega_{3}} \right)} \right\rbrack s_{12}} + {C_{pa}{\ln\left( \frac{T_{4}}{T_{3}} \right)}} - {R_{a}{\ln\left( \frac{P_{a4}}{P_{a3}} \right)}} + {\omega_{4}s_{v4}} - {\omega_{3}s_{v3}} - {\frac{m_{l1}}{m_{a}}s_{l1}}}},} & (5)\end{matrix}$where ω is the humidity ratio, C_(p) is the specific heat, R is theengineering gas constant, and P_(a) is the partial pressure of air.

The control volume formulation of energy conservation applied to theadiabatic diffusion tower leads to,m _(l1) h _(l1) +m _(a) h _(a3) +m _(v3) h _(v3) −m _(l2) h _(l2) −m_(a) h _(a4) −m _(v4) h _(v4)=0  (6)where h denotes the enthalpy. The enthalpy of the brine exiting thediffusion tower is obtained from Eqs. (6) and (4) as, $\begin{matrix}{{{h_{12}\left( T_{2} \right)} = \frac{{\frac{m_{l1}}{m_{a}}h_{l1}{C_{pa}\left( {T_{4} - T_{3}} \right)}} + {\omega_{3}h_{v3}} - {\omega_{4}h_{v4}}}{\frac{m_{l1}}{m_{a}} - \left( {\omega_{4} - \omega_{3}} \right)}},} & (7)\end{matrix}$and the brine temperature (T₂) is evaluated from the enthalpy. The ratioof the feed water to air flow through the diffusion tower,$\frac{m_{l1}}{m_{a}},$is another controlling variable in the analysis. For all computationsthe feed water flow rate will be fixed at 1 kg/s while the air flow ratewill be varied.

The humidity ratio entering the diffusion tower, ω₃, is determined byrecognizing that it is the same as the humidity ratio exiting thecondenser, where T₇ is 15° C. Prior to entering the diffusion tower, theair/vapor mixture is convectively heated by the ambient as it is pumpedback to the diffusion tower. This may be achieved by placing fins on thereturn line to the diffusion tower. Taking the ambient temperature to be25° C., it follows that T₃=25° C. and ω₃=ω₇.

The first case considered is where there is no heating in the mainheater. The desalination process is entirely driven by the difference intemperature of the feed water drawn at shallow depths and the coolingwater drawn at more substantial depth. In this case, T_(H)/T_(L)=1.04.FIG. 4 shows the rate of entropy generation within the diffusion towerand the brine temperature exiting the diffusion tower for a locus ofpossible operating conditions. It is observed that the second lawthermodynamics is satisfied for the entire parametric range considered.At the highest air to feed water flow ratio more fresh water productionis possible, but there is a lower limit beyond which the exit brine willfreeze.

FIG. 5 shows the brine temperature (T₂) exiting the diffusion tower as afunction of the exit air temperature from the diffusion tower (T₄) forthe same locus of operating conditions as in FIG. 4. It is advantageousto have a high air temperature leaving the diffusion tower so that thehumidity ratio and fresh water production rate are as high as possible.For this case the exit air temperature is primarily constrained by theinlet feed water temperature (T₁). Due to heat transfer considerationsit would be impractical to design the diffusion tower such that T₄exceeds T₁. Thus in this analysis, the exiting air temperature from thediffusion tower does not exceed the inlet feed water temperature.

FIG. 6 shows the ratio of fresh water production rate to the inlet feedwater rate as a function of the exit air temperature for different airto feed water flow ratios. Clearly, the production rate increases withincreasing exit air temperature and increasing air to feed water flowratios. However, both these parameters are constrained, and for the caseof no heating of the feed water (T_(H)/T_(L)=1.04), the maximum freshwater production efficiency (m_(fw)/m₁₁) is approximately 0.035.

The next cases considered are where the diffusion tower inlet watertemperatures are 60° C. and 80° C. which correspond to T_(H)/T_(L)=1.17and 1.23, respectively. FIGS. 7 and 8 show the rate of entropygeneration in the diffusion tower for T_(H)/T_(L)=1.17 and 1.23,respectively. Again the second law of thermodynamics is satisfied forthe entire parametric range considered. The entropy generation tends tobe lower for lower air to feed water flow ratios and higher exit brinetemperatures. At higher air to feed water flow ratios, the constraint isthat the brine does not freeze. FIG. 9 shows the range of possible exitbrine temperatures and exit air temperatures for different air to feedwater flow ratios when the diffusion tower inlet water temperature is60° C. (T_(H)/T_(L)=1.17). FIG. 10 shows the range of temperatures whenthe diffusion tower inlet water temperature is 80° C.((T_(H)/T_(L)=1.23). The maximum fresh water production will occur withas high an exit air temperature as possible. In order to satisfy anenergy balance on the diffusion tower, the exit brine temperaturedecreases with increasing exit air temperature. In contrast to the casewith no heating, the exit air temperature is primarily constrained bythe fact that the brine cannot freeze, especially at higher air to feedwater flow ratios. At very low air to feed water flow ratios andT_(L)=60° C. and 80° C., the exit air temperature is generallyconstrained by the inlet water temperature.

For respective diffusion tower inlet water temperatures of 60° C. and80° C., FIGS. 11 and 12 show the ratio of fresh water production to theinlet feed water flow rate as a function of the exit air temperature fordifferent air to feed water flow ratios. It is observed that the freshwater production efficiency increases with increasing exit airtemperature and increasing air to feed water flow ratio. The maximumfresh water production efficiency for T₁=60° C. is approximately 0.08,while that for T₁=80° C. is approximately 0.11. Therefore, one advantageof in creasing the diffusion tower inlet water temperature is that thefresh water production efficiency increases.

For respective diffusion tower inlet water temperatures of 60° C. and80° C., FIGS. 13 and 14 show the energy consumed per unit of fresh waterproduction as a function of exit air temperature for different air tofeed water flow ratios over the entire parameter space considered.Although, details of the low energy consumption regime are difficult todiscern, it is interesting to observe that increasing both the exittemperature and the air flow results in a reduced rate of energyconsumption. In order to explore the lower energy consumption regimeFIGS. 15 and 16 have been prepared for diffusion tower inlet watertemperatures of 60° C. and 80° C., respectively.

For T₁=60° C. the lower limit on energy consumed per unit of fresh waterproduction is about 0.06 kWh/kg_(fw) while that for T₁=80° C. isapproximately 0.05 kWh/kg_(fw). In this analysis the energy consumptiondue to pumping is neglected, and the current results suggest that theenergy consumption is lower with higher air to feed water flow ratios.However, with higher air flow, the pumping power required will increaseas well. Therefore, it is expected that in actual practice there is someminimum energy consumption associated with a specific air to feed waterflow ratio that is less than the maximum flow. The inclusion of thepumping power in the overall analysis is the subject of a futureinvestigation.

It is also of interest to compare the fresh water production and energyconsumption between the cases of T₁=60° C. and 80° C. There is onlymarginal improvement in the fresh water production and energyconsumption when increasing T_(H) from 60° C. to 80° C. Thisdemonstrates that the DDD process is best suited for applications wherethe waste heat driving the process has low thermodynamic availability.

A pilot plant was designed for a fresh water production rate of 10,000gallons per day using the system and associated process shownschematically in FIG. 17. The design flow rate and temperatures for thepilot plant are listed in the table below. Note that humidity isdimensionless (mass of water vapor/mass of air).

Table Design parameters for pilot fresh water production plant WaterCooled Air Cooled Fresh Water Fresh Water Condenser Condenser Air CoolerCooler Air flow rate (kg/s) 10.2 20.4 Steam Flow (lb/hr) 894 894 Inletair temperature (C.) 25 25 Exit air temperature (C.) 50 30 Inlet freshwater temperature (C.) 37.2 27 Exit fresh water temperature (C.) 27 26Mineralized water in (C.) 26 25 Mineralized water out (C.) 50 26Diffusion Tower Direct Contact Condenser Mineralized water flow (kg/s)4.1 Mineralize water in (C.) 50 Brine water out (C.) 31 Air flow rate(kg/s) 10.2 10.2 Inlet air temp (C.) 50 41.3 Exit air temp (C.) 41.3 26Fresh water flow (kg/s) 20.4 Humidity in 0.023 0.053 Humidity out .053.023

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description as well as the examples which follow are intendedto illustrate and not limit the scope of the invention. Other aspects,advantages and modifications within the scope of the invention will beapparent to those skilled in the art to which the invention pertains.

1. An apparatus for purifying water, comprising: a source of a heatedair stream, said heated air stream having a temperature above an ambienttemperature; a diffusion tower having high surface area material thereinfor receiving a water stream including at least one impurity andcreating at least one region having thin films of water therein fromsaid water stream, wherein said heated air stream is directed over saidthin films of water to create a humidified air stream that is at leastsubstantially saturated, and at least one direct contact condenser influid communication with said humidified air stream for condensing saidhumidified air stream, wherein purified water is produced.
 2. Theapparatus of claim 1, wherein energy to heat said heated air stream isobtained from waste heat from low pressure condensing steam from a powerplant, waste heat from a combustion engine, geothermal heat, or solarenergy.
 3. The apparatus of claim 2, wherein said waste heat is providedin the form of low pressure waste steam, said waste steam directed to anair cooled condenser, wherein said air cooled condenser transfers heatfrom said steam to heat air to generate said heated air stream.
 4. Theapparatus of claim 3, further comprising a water cooled condenser,wherein said water cooled condenser receives and further cools saidwaste steam after cooling at said air cooled condenser.
 5. The apparatusof claim 4, wherein said water stream is fluidly connected and providescooling water for said water cooled condenser, said water stream exitingsaid water cooled condenser as a heated water stream having atemperature above ambient temperature.
 6. The apparatus of claim 1,further comprising a fresh water storage tank for storing said purifiedwater, wherein a portion of said purified water is supplied to saiddirect contact condenser for condensing said humidified air stream. 7.The apparatus of claim 1, wherein said direct contact condenser includesa water sprayer, wherein said direct contact condenser condenses saidhumidified air stream through contact with a water spray.
 8. A powerplant including a water purification system, comprising: an apparatusfor converting a source of energy into electrical energy and providingwaste heat; a heat exchanger for heating an air stream to form a heatedair stream using said waste heat; said heated air stream having atemperature above an ambient temperature; a diffusion tower having highsurface area material for receiving a water stream including at leastone impurity and creating at least one region having thin films of watertherein from said water stream, wherein said heated air stream isdirected over said thin films of water to create a humidified air streamthat is at least substantially saturated, and at least one directcontact condenser in fluid communication with said humidified air streamfor condensing said humidified air stream, wherein purified water isproduced.
 9. The power plant of claim 8, wherein said water stream isdrawn from sea or brackish water, wherein said water purificationcomprises desalinization.
 10. The power plant of claim 8, wherein saidsource of energy comprises fossil fuel or nuclear fuel.
 11. The powerplant of claim 8, wherein said waste heat is provided in the form of lowpressure waste steam, said waste steam directed to an air cooledcondenser, wherein said air cooled condenser transfers heat from saidsteam to heat air to generate said heated air stream.
 12. The powerplant of claim 11, further comprising a water cooled condenser, whereinsaid water cooled condenser receives and further cools said waste steamafter cooling at said air cooled condenser.
 13. The power plant of claim12, wherein said water stream is fluidly connected and provides coolingwater for said water cooled condenser, said water stream exiting saidwater cooled condenser as a heated water stream having a temperatureabove ambient temperature.
 14. The power plant of claim 11, furthercomprising a fresh water storage tank for storing said purified water,wherein a portion of said purified water is supplied to said directcontact condenser for condensing said humidified air stream.
 15. Thepower plant of claim 11, wherein said direct contact condenser includesa water sprayer, wherein said direct contact condenser condenses saidhumidified air stream through contact with a water spray.
 16. A methodfor purifying water, comprising the steps of: providing a water streamincluding at least one non-volatile impurity; spraying said heated waterstream into a diffusion tower having a high surface area materialtherein, wherein thin films of water form on surfaces of said highsurface area material; forcing a heated air stream over said thin filmsof water, wherein water from said thin films of water evaporate anddiffuse into said air stream to create a humidified air stream, anddirect contact condensing said humidified air stream, wherein purifiedwater is produced.
 17. The method of claim 16, wherein said directcontact condensing comprises contacting said humidified air stream witha water spray.
 18. The method of claim 16, wherein heat to heat saidheated air stream is obtained from waste heat from low pressurecondensing steam from a power plant, waste heat from a combustionengine, geothermal heat or solar energy.
 19. The method of 16, whereinsaid water stream is drawn from a sea or brackish water, wherein saidpurifying water comprises desalinization.
 20. The method of claim 16,wherein said waste heat is provided in the form of low pressure wastesteam, further comprising the step of directing said waste steam to anair cooled condenser, wherein said air cooled condenser transfers heatfrom said steam to heat air to generate said heated air stream.
 21. Themethod of claim 20, further comprising the step of cooling said wastesteam after cooling at said air cooled condenser using a water cooledcondenser.
 22. The method of claim 16, wherein said direct contactcondenser includes a water sprayer, wherein said direct contactcondensing comprises condensing said humidified air stream throughcontact with a water spray.